Liquid container bags, such as bags for drinks, or drainage bags, such as urine bags, blood bags and the like, are usually made from two superposed plastic films, which are welded together along their periphery. For providing the bag with an outlet channel it is common to weld a tube between the two plastic films to provide communication via the tube between the interior and the exterior of the bag. One of the plastic films is thus welded on to one half of the periphery of the tube and the other plastic film is welded on to the other half of the periphery of the tube. The two plastic films are welded together on both sides of the tube. However, optionally, the films may only be welded to the tube at the transition areas on each side of the tube, the films being drawn tightly over the periphery of the tube to avoid leakage. Optionally, an adhesive may be provided between the films and the periphery of the tube.
In practice it has proved difficult to obtain a liquid-tight connection between the outer face of the tube and the plastic films in the two transition areas in which the outer face of the tube and the two plastic films meet. Even though two welding jaws are used, which each in a cross-sectional view has a semi-circular shape and jointly in closed position define a circular opening corresponding to the external diameter of the tube, leaks often arise in the mentioned areas at the transition from the film-tube attachment to the film-film attachment.
A further challenge is aligning the seal around the tube with a given feature on the plastic film so as to form a liquid proof connection between the tube seal and the given feature. For example, in the production of a liquid container, the seal around a tube may need to be brought into overlapping alignment with a sealing seam of a pre-fabricated liquid container bag into which the tube is to be mounted. Such further process specifications/constraints/requirements that have to be fulfilled by the tube sealing makes it increasingly difficult to find a window of process parameters that produces an overall satisfactory tube seal when using known sealing jaws/processes.
EP 1 677 966 B1 discloses a method and apparatus for tube welding, wherein the tube is arranged between two parallel plastic films. The plastic films are welded by means of welding jaws to the outer face of the tube and welded together in the area adjoining the tube to form a joint face. The tube is deformed during welding by means of a compressive force applied by the welding jaws to provide the outer surface of the tube with a substantially plane portion extending on both sides of the joint face and substantially perpendicular thereto.
Despite the improvements achieved by the method and apparatus according to EP 1 677 966 B1, problems may still arise in the above-mentioned transition areas, in particular when trying to satisfy the further constraints of the tube welding process, i.e. provide satisfactory results for welding the plastic films together and/or sealing the plastic films to the tube and/or aligning the tube seal with given features. The problem of simultaneously satisfying the process constraints for the film-to-film attachment, the film-to-tube attachment and for the transition areas in one ultrasonic process becomes even more pronounced for thin plastic films with a thickness well below 0.15 mm. Problems observed in practice include excessive hardening of the film, burning and cutting of the film, unsatisfactory attachment of the film to the tube, and leaks particularly along the tube in the two transition areas in which the outer face of the tube and the two plastic films meet.
Other known apparatus and methods for sealing a tube into an opening of a bag formed by plastic film address the problem of providing a seal in the transition area between the film-film attachment and the film-tube attachment by partially melting and moulding the material of the tube so as to fill the critical gap. Examples for such solutions requiring the melting and deformation of the tube material and/or protrusions provided thereon may be found for example in JP 2 791 387 or DE 29 612 778 U1. Common for these solutions is that a recess or chamfer/bevel is provided in the portion of the welding jaws where the plastic film is welded to tube material, i.e in the film-tube portion of the respective welding jaws, wherein the tube material has been melted and moulded according to the shape of that recess during the film-to-tube welding process. Such a sealing process thus requires at least a partial melting of the tube material and has the disadvantage of imposing narrow restrictions on the choice of process compatible tube/film materials. Furthermore, the partial melting of the tube implies in many cases a softening of the tube structure and may require the use of stiffening means inside the tube, such as a mandrel in order to avoid a collapse of the tube during the welding operation. This adds to process complexity and cost.